Abstract

Using a constellation of several hundred low-Earth-orbit satellites--a
global, broadband "Internet-in-the-sky," Teledesic will enable
affordable access to fiber-like telecommunications capability anywhere in
the world. The Teledesic Network will allow local service providers to
extend their networks in terms of both scope of services and geographic
reach. It will be a local service provided through a global network.

Access to information is becoming increasingly essential to all those
things we associate with quality of life: economic opportunity, education,
health care, and public services. Yet, most people and places in the world
do not now have access even to basic telephone service. Even those who do
have access to basic phone service get it through 100-year-old
technology--analog copper wire networks--that for the overwhelming part
will never be upgraded to an advanced digital capability.

While many places in the world are connected by fiber--and the number of
places is growing--it is used primarily to connect countries and telephone
company central offices. Even in the most developed countries, high cost
will prevent the deployment of a significant amount of fiber for local
access to most individual offices and homes. In most parts of the world,
fiber deployment in the local access network likely never will happen.

This lack of broadband local access is a major problem for all of the
world's societies. If these powerful technologies are available only in
advanced urban areas, people will be forced to migrate to those areas in
search of economic opportunity and to fulfill other needs and desires. It
is no longer sound--economically or environmentally--to force people to
migrate to increasingly congested urban areas in search of opportunity. The
real potential of the information age is to find a means of allowing people
to choose where they live and work based on things like family, community,
and quality of life rather than access to infrastructure.

The one-way information dissemination made possible through broadcast
technologies has created a means for nearly the entire world to view the
benefits of advanced technology. But having created a means for everyone to
see all the benefits of our societies we have also created
expectations--legitimate expectations--that will seek fulfillment.
Increasingly, even a sole proprietor in the developing world will need the
same kind of connection to the "Global Village" available now
only to the biggest, richest corporations. Through schools, community
centers, and home access, individuals are beginning to use broadband
connections for services such as Internet access, telemedicine, distance
learning, videoconferencing, telecommuting, and many other applications. We
need to create the two-way network links that allow people to participate
economically and culturally with the world at large without requiring that
they pick up and move to places with modern telecommunications
infrastructure.

For more than three decades, geostationary satellites have been
virtually the exclusive means of providing commercial space-based
communications. Geostationary satellites will continue to play an important
role, particularly for broadcast applications. However, these systems have
a number of limitations for two-way communications, such as the need for
high-power terminals and the signal delay caused by their high altitude.
This delay means that a large number of applications, including essential
Internet technologies such as the World Wide Web, are adversely
affected--or simply don't work--over geostationary satellites. Because of
their delay, geostationary satellites can never provide fiber-like delays
to be seamlessly compatible with fiber-based networks on the ground. For
natural economic reasons, these systems also tend to focus their capacity
on the more economically developed areas. Via Satellite recently
reported, for instance, that of over 200 geostationary commercial
communications satellites, only one is on order to provide service
to Africa.

New options are becoming available, however, with the development of
non-geostationary communication systems, which primarily use
low-Earth-orbit (LEO) satellites. LEO satellite systems can help meet the
demand for information by providing global access to the telecommunications
infrastructure currently available only in advanced urban areas of the
developed world. The low altitude of LEO systems allows them to provide
delays that are seamlessly compatible with terrestrial networks. Just as
networks on the ground have evolved from centralized systems built around a
single mainframe computer to distributed networks of interconnected PCs,
space-based networks are evolving from centralized networks relying on a
single geostationary satellite to distributed networks of interconnected
low-Earth-orbit satellites.

The evolution from geostationary to LEO satellites has resulted in a
number of proposed global satellite systems, which can be grouped into
three distinct types. These LEO systems can best be distinguished by
reference to their terrestrial counterparts: paging, cellular, and fiber.

Using a constellation of several hundred LEO satellites, Teledesic will
enable affordable access to fiber-like telecommunications services to
institutions and individuals anywhere in the world. This ability to deliver
fiber-like, broadband, digital transmission capability at low cost,
regardless of location, distinguishes the Teledesic Network from other
existing and proposed communications systems.

Teledesic was founded in 1990 and is headquartered in Kirkland,
Washington, a suburb of Seattle. Teledesic's principal shareholders are
Craig O. McCaw and William H. Gates III. Mr. McCaw, who leads the company
as its chairman, is the founder of McCaw Cellular Communications, which he
built into the world's largest wireless communications company before its
1994 merger with AT&T. Mr. Gates is the co-founder, chairman, and CEO
of Microsoft Corporation, the world's largest computer software company.

At the 1995 World Radio Conference, Teledesic received support from the
developed and developing world alike, resulting in a new international
satellite service designation for the frequencies necessary to accommodate
the Teledesic Network. The action of the World Radio Conference mirrors
Teledesic's success in obtaining a similar designation from the U.S.
Federal Communications Commission (FCC). Teledesic is well positioned for
FCC licensing in the near future.

Teledesic plans to begin service in the year 2002. Teledesic does not
intend to market services directly to end users. Rather, it will provide an
open network for the delivery of such services by others. The Teledesic
Network will enable local telephone companies and government authorities in
host countries to extend their networks, both in terms of geographic scope
and in the kinds of services they can offer. Ground-based gateways will
enable service providers to offer seamless links to other wireline and
wireless networks.

Teledesic uses small, "Earth-fixed" cells both for efficient
spectrum utilization and to respect countries' territorial boundaries.
Within a 53 by 53 km cell, the network will be able to accommodate over
1,800 simultaneous 16 Kbps voice channels, 14 simultaneous E-1 (2 Mbps)
channels, or any comparable combination of channel bandwidths. This
represents a significant system capacity, equivalent to 20,000 simultaneous
E-1 lines worldwide, with the potential for graceful growth to higher
capacities. The network offers high-capacity "bandwidth on
demand" through standard user terminals. Channel bandwidths are
assigned dynamically and asymmetrically and range from a minimum of 16 kbps
up to 2 Mbps on the uplink, and up to 28 Mbps on the downlink. Teledesic
will also be able to provide a smaller number of high-rate channels at 155
Mbps to 1.2 Gbps for gateway connections and users with special needs. The
low orbit and high frequency (30 GHz uplink/20 GHz downlink) allow the use
of small, low-power terminals and antennas, with a size and cost comparable
to a notebook computer.

The Teledesic constellation design supports the network requirements for
quality, capacity, and integrity. To provide high-quality, high-speed
wireless channels at the intended peak-user density levels requires
substantial bandwidth. The only feasible frequency band internationally
allocated to fixed satellite service that meets Teledesic's requirements is
the Ka band. High rain attenuation, terrain blocking, and other terrestrial
systems in this band make it difficult for earth terminals to communicate
reliably with a satellite at a low elevation angle. The Teledesic
constellation uses a high elevation mask angle to mitigate these problems.
A low orbit altitude is used to meet the requirements for low end-to-end
delay and reliable communication links that use low power and small
antennas. The combination of low altitude and high elevation angle results
in a small coverage area per satellite and a large number of satellites for
global coverage. A high degree of coverage redundancy and the use of
on-orbit spares support the network reliability requirements.

One or more local service providers in the United States and in each
host country will serve end users. User terminals communicate directly with
Teledesic's satellite-based network to other terminals and through gateway
switches to other networks, such as the Public Switched Telephone Network
and the Internet.

The network uses fast packet switching technology, with a packet design
similar to asynchronous transfer mode (ATM). All communication is treated
identically within the network as streams of short fixed-length packets.
Each packet contains a header that includes address and sequence
information, an error-control section used to verify the integrity of the
header, and a payload section that carries the digitally encoded voice or
data. Conversion to and from the packet format takes place in the
terminals. The fast packet switch network combines the advantages of a
circuit-switched network (low delay "digital pipes") and a
packet-switched network (efficient handling of multi-rate and bursty data).
Fast packet switching technology is ideally suited for the dynamic nature
of a LEO network.

Each satellite in the constellation is a node in the fast packet switch
network and has intersatellite communication links with eight adjacent
satellites. Each satellite is normally linked with four satellites within
the same plane (two in front and two behind) and with one in each of the
two adjacent planes on both sides. This interconnection arrangement forms a
non-hierarchical "geodesic," or mesh, network and provides a
robust network configuration that is tolerant to faults and
local congestion.

The topology of a LEO-based network is dynamic. Each satellite keeps the
same position relative to Ether satellites in its orbital plane. Its
position and propagation delay relative to earth terminals and to
satellites in other planes change continuously and predictably. In addition
to changes in network topology, as traffic flows through the network,
queues of packets accumulate in the satellites, changing the waiting time
before transmission to the next satellite. All of these factors affect the
packet routing choice made by the fast packet switch in each satellite.
These decisions are made continuously within each node using Teledesic's
distributed adaptive routing algorithm. This algorithm uses information
transmitted throughout the network by each satellite to "learn"
the current status of the network in order to select the path of least
delay to a packet's destination. The algorithm also controls the connection
and disconnection of intersatellite links.

The network uses a "connectionless" protocol, similar to the
routing of the Internet Protocol (IP). Packets of the same connection may
follow different paths through the network. Each node independently routes
the packet along the path that currently offers the least expected delay to
its destination. The required packets are buffered, and if necessary
resequenced, at the destination terminal to eliminate the effect of timing
variations. Teledesic has performed extensive and detailed simulation of
the network and adaptive routing algorithm to verify that they meet
Teledesic's network delay and delay variability requirements.

All of the Teledesic communications links transport data and voice as
fixed-length (512 bit) packets. The basic unit of channel capacity is the
"basic channel," which supports a 16 kbps payload data rate and
an associated 2 kbps "D-channel" for signaling and control. Basic
channels can be aggregated to support higher data rates. For example, eight
basic channels can be aggregated to support the equivalent of a 2B + D
Integrated Services Digital Network (ISDN) link, or 97 channels can be
aggregated to support an equivalent T-1 (1.544 Mbps) connection. A
Teledesic terminal can support multiple simultaneous network connections.
In addition, the two directions of a network connection can operate at
different rates.

The links are encrypted to guard against eavesdropping. Terminals
perform the encryption/decryption and conversion to and from the packet
format. The uplinks use dynamic power control of the radio frequency
transmitters so that the minimum amount of power is used to carry out the
desired communication. Minimum transmitter power is used for clear sky
conditions. The transmitter power is increased to compensate for rain.

The Teledesic Network accommodates a wide variety of terminals and data
rates. Standard terminals will include both fixed-site and transportable
configurations that operate at multiples of the 16 kbps basic channel
payload rate up to 2.048 Mbps (the equivalent of 128 basic channels). These
terminals can use antennas with diameters from 16 cm to 1.8 m as determined
by the terminal's maximum transmit channel rate, climatic region, and
availability requirements. Their average transmit power varies from less
than 0.01 W to 4.7 W depending on antenna diameter, transmit channel rate,
and climatic conditions. All data rates, up to the full 2.048 Mbps, can be
supported with an average transmit power of 0.3 W by suitable choice of
antenna size.

Within its service area, each satellite can support a combination of
terminals with a total throughput equivalent to over 125,000 simultaneous
basic channels.

The network also supports a smaller number of fixed-site GigaLink
Terminals that operate at the OC-3 rate (155.52 Mbps) and multiples of this
rate up to OC-24 (1.2 Gbps). Antennas for these terminals can range in size
from 28 cm to 1.6 m as determined by the terminal's maximum channel rate,
climatic region, and availability requirements. Transmit power will range
from 1 W to 49 W depending on antenna diameter, data rate, and climatic
conditions. Antenna site-diversity can be used to reduce the probability of
rain outage in situations where this is a problem.

GigaLink Terminals provide gateway connections to public networks and to
Teledesic support and data base systems including Network Operations and
Control Centers (NOCCs) and Constellation Operations Control Centers
(COCCs), as well as to privately owned networks and high-rate terminals. A
satellite can support up to sixteen GigaLink terminals within its service
area.

Intersatellite links (ISLs) interconnect a satellite with eight
satellites in the same and adjacent planes. Each ISL operates at 155.52
Mbps, and at multiples of this rate up to 1.24416 Gbps depending upon the
instantaneous capacity requirement.

One benefit of a small satellite footprint is that each satellite can
serve its entire coverage area with a number of high-gain scanning beams,
each illuminating a single small cell at a time. Small cells allow
efficient reuse of spectrum, high channel density, and low transmitter
power. However, if this small cell pattern swept the Earth's surface at the
velocity of the satellite (approximately 25,000 km per hour), a terminal
would be served by the same cell for only a few seconds before a channel
reassignment or "hand-off" to the next cell would be necessary.
As in the case of terrestrial cellular systems, frequent hand-offs result
in inefficient channel utilization, high processing costs, and lower system
capacity. The Teledesic Network uses an Earth-fixed cell design to minimize
the hand-off problem.

The Teledesic system maps the Earth's surface into a fixed grid of
approximately 20,000 "supercells," each consisting of nine cells.
Each supercell is a square 160 km on each side. Supercells are
arranged in bands parallel to the Equator. There are approximately 250
supercells in the band at the Equator, and the number per band decreases
with increasing latitude. Since the number of supercells per band is not
constant, the north-south supercell borders in adjacent bands are not
aligned.

A satellite footprint encompasses a maximum of 64 supercells, or 576
cells. The actual number of cells for which a satellite is responsible
varies by satellite with its orbital position and its distance from
adjacent satellites. In general, the satellite closest to the center of a
supercell has coverage responsibility. As a satellite passes over, it
steers its antenna beams to the fixed cell locations within its footprint.
This beam steering compensates for the satellite's motion as well as the
Earth's rotation. (An analogy is the tread of a bulldozer that remains in
contact with the same point while the bulldozer passes over).

Channel resources (frequencies and time slots) are associated with each
cell and are managed by the current "serving" satellite. As long
as a terminal remains within the same Earth-fixed cell, it maintains the
same channel assignment for the duration of a call, regardless of how many
satellites and beams are involved. Channel reassignments become the
exception rather than the norm, thus eliminating much of the frequency
management and hand-off overhead.

A database contained in each satellite defines the type of service
allowed within each Earth-fixed cell. Small fixed cells allow Teledesic to
avoid interference to or from specific geographic areas and to contour
service areas to national boundaries. This would be difficult to accomplish
with large cells or cells that move with the satellite.

Teledesic's engineering effort builds on previous work done in many
advanced commercial and government satellite programs and was assisted by
several government laboratories. The Teledesic system uses proven
technology and experience from many U.S. defense programs, including the
"Brilliant Pebbles" program, which was conceived as a similar
orbiting global constellation of 1,000 small, advanced, semi-autonomous,
interconnected satellites. Since 1990, Teledesic has drawn on the expertise
of the contractors on that and many other programs for input into the early
system design activities.

Design, construction, and deployment costs of the Teledesic Network are
estimated at $9 billion. The Teledesic Network represents the first time
that satellites and their associated subsystems will be designed and built
in quantities large enough to be mass-produced and tested. These
substantial economies of scale enable a cost structure comparable to that
of wireline service in advanced urban areas.

The Teledesic Network emulates the most famous distributed network, the
Internet, while adding the benefits of real-time connections,
location-insensitive access, and broadband-on-demand capability. Because
their low altitude eliminates the delay associated with traditional
geostationary satellites, these networks can provide communications that
are seamlessly compatible with terrestrial, fiber-based standards.

Because LEO satellites move in relation to the Earth, they all share a
characteristic with profound implications: Continuous coverage of any point
on Earth requires--in effect, global coverage. In order to provide service
to the advanced markets, the same quality and quantity of capacity has to
be provided to the developing markets, including those areas to which it
would not be economically feasible to provide that kind of capacity for its
own sake. In this sense, LEO satellite systems represent an inherently
egalitarian technology that promises to radically transform the economics
of telecommunications infrastructure to enable universal access to the
Information Age.